The placing of hyperextensive or hyperflexive pressure on joints–especially of the hands, evoking a cracking sound.
Conventional wisdom has long held that knuckle cracking either causes arthritis of the hand or exacerbates it. Not true. Dr Donald L Unger, MD, received a highly coveted 2009 Ig Nobel prize for his single author, single patient (himself) contribution to knuckle-ology by cracking the joints of his left hand–but not his right–for 50-plus years.
No arthritis in either.
Reference Unger DL. Arthritis Rheum. 1998 May;41(5):949-50, later updated